Techniques are being developed to obtain DNA probes specific to various regions of the genome of the endogenous baboon virus. These probes should allow DNA hybridization measurements to determine which viral genes are most conserved both between species and within the multiple copies of viral related sequence in the baboon. Probes specific to the most conserved regions of the viral genome may provide improved sensitivity for endogenous human viral sequences. The virus PP-REC, thought to be a recombinant virus containing baboon, mink, mouse and rat viral information, is also being studied in order to determine which portions of the viral genome are specified by each species of viral information.